1. Field of the Invention
The present invention relates to thermoforming generally and, more particularly, but not by way of limitation, to novel pressure box and method for thermoforming, which box is adjustable in two dimensions.
2. Background Art
Thermoforming is widely used for producing three dimensional objects from relatively thin sheets of thermoplastic materials, such objects often being used a containers or parts of containers for goods, and are typified by blister pack covers, pastry domes, and fast-food sandwich containers, to name only a few examples.
In a typical thermoforming process, a rectangular mold base is provided which may have one or more cavities defined therein or may have provision made for affixing thereto one or more mold cavities. A web of thermoplastic material is fed from a roll of such material, heated and then indexed over the mold cavities. A plurality of holes defined through the walls of the cavities is connected to a vacuum source to draw the thermoplastic material into the cavities. To assist this drawing operation, pressure is applied to the upper surface of the thermoplastic material. Such pressure is supplied from a rectangular pressure box which is temporarily placed over the mold base and removably sealed thereto, the pressure box having an open base and a top with four walls depending therefrom. After forming, coolant may be supplied to the mold base and the molded objects are removed from the mold base typically by means of a stripper plate built into the mold base.
In high volume molding operations, the mold base and the pressure box are often dedicated equipment items of fixed dimensions. In custom thermoforming operations, however, it is common to use mold bases having different dimensions for different molding runs. In order to accommodate such mold bases, pressure boxes have been available which are adjustable in the index direction; that is, the direction in which the web of thermoplastic material moves. This is accomplished by having two parallel side walls and one end wall attached to the top wall of the pressure box, with the other of the end walls selectively positionable in the index direction and removably attached to the side walls and the top wall.
Such pressure boxes adjustable in the index direction have been available for the past 20 to 30 years. The availability of pressure boxes adjustable in the index direction permits accommodating a variety of mold bases having different dimensions in the index direction, thus reducing the inventory of pressure boxes which must be maintained by a custom thermoformer. However, heretofore, no pressure box has been adjustable in the direction orthogonal to the index direction and sometimes a custom thermoformer may have to inventory three or more pressure boxes of different widths. Having available such a pressure box adjustable in two dimensions would greatly decrease the number of pressure boxes required to be inventoried by a custom thermoformer and, perhaps, a custom thermoformer might require only one such pressure box, with the concomitant cost savings.
Accordingly, it is a principal object of the present invention to provide a thermoforming pressure box adjustable in both the index direction and the direction orthogonal to the index direction.
It is a further object of the invention to provide such a pressure box that is economically constructed and conveniently adjusted.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.